1. Field of the Invention
The present invention relates to a duty discriminating circuit, and more specifically to a circuit for discriminating a duty (a rate of a precedent high-level portion to one period of an input signal whose level changes periodically) on the basis of a threshold level.
2. Description of the Prior Art
A conventional duty discriminating circuit will be described hereinbelow by taking the case of the one used for a household VTE (Video Tape Recorder).
In household VTRs based on the VHS standard, there exists cases where various codes such as index codes, address codes, etc. represented by bit information are recorded on a video tape. As a method of recording the bit information on the video tape, a method of using a control signal recorded on the video tape for phase matching has been so far known. In this method, the recorded one-bit information or data "0" and "1" are represented by two one-cycle pulses of two different duties, respectively. In more detail, in usual, the duty of "0" is determined to be 80% and the duty of "1" is determined to be 27.5%. Further, whenever these bit information or data are reproduced, the "0" or "1" of the reproduced signal has been so far discriminated by comparing the duty of the reproduced control signal with a threshold value (described later) with the use of a duty discriminating circuit.
In more detail, in the conventional duty discriminating circuit, a counter is activated in response to a rise edge of a signal to be discriminated; the precedent portion of the signal is counted; a rate of the counted number of the precedent portion to a count number corresponding to one period of the signal is determined to be a duty of the signal; and the determined rate is compared with a threshold value of 50%.
In the conventional duty discriminating circuit, therefore, it is ideal that the threshold value is determined to be an intermediate value between the two duties used to record the hit information as follows: EQU (27.5+60)/2-43.5%
Furthermore, in the conventional duty discriminating circuit, when the video tape is rotated in the reverse direction, since the precedent portion and the subsequent portion are reversed with respect to each other, the duties of the reproduced control signal become 72.5% and 40%, respectively, so that the ideal threshold value changes as follows: EQU (27.5+40)/2=56.25%
As described above, since the threshold value (i.e., 50%) adopted in the conventional duty discriminating circuit deviates largely from an ideal value, when the video tape is fed at a high speed, there exists a problem in that an erroneous discrimination occurs, in particular when the reproducibility of the reproduced control signal is low.
Further, in the present VTR, although the duties of 27.5% and 60% are adopted in general, since there exists a possibility that other duties will be adopted in the future, it is preferable to use a duty discriminating circuit by which a threshold value other than 50% can be adopted.
To overcome the above-mentioned problem, Japanese Laid-open (Kokai) Patent Application No. 1-282919 discloses a duty discriminating circuit which can discriminate the duties on the basis of and given threshold values by using two different clock signals separately in the precedent high-level portion and the subsequent low-level portion. In this prior art duty discriminating circuit, however, since complicated calculations are required for the duty discrimination, there exists another problem in that the circuit scale is inevitably increased.